The present invention relates to the biodegradation of a liquid scintillation cocktail (LSC) having high concentrations of both organic solvents and emulsifiers. In a preferred embodiment, the biodegradation is effected by a microorganism identified as Pseudomonas sp NRRL B-18435. A deposit of this microorganism has been made in the Northern Regional Research Center, U.S. Department of Agriculture, in Peoria, Ill., and is available under conditions set forth under 37 CFR.
LSC waste solutions are' generated in a broad range of medical and biotechnological environments. For instance, when it is desired to trace a particular molecule or compound through a metabolic pathway, it is frequently useful to tag the compound or molecule under consideration with a isotope, such as tritium, C-14, S-35 or P-32. When analyzing the end result of the experiment, the presence (or quantity) of the tagged molecule can be detected by a liquid scintillation counter. Such counters detect fluorescence of introduced chemicals (fluor) resulting from the low energy beta particle emission from the tagged molecule. This reaction occurs in a "liquid scintillation cocktail", or LSC, which typically contains one of the following solvents: toluene, p-xylene, 1, 2, 4-trimethyl benzene (pseudocumene), benzene, dioxane or cyclohexane. Other organic solvents may be found in the LSC.
Waste LSC solutions containing any of the above solvents are considered mixed hazardous wastes because they contain both radioactive and toxic organic compounds. Under current Environmental Protection Agency (EPA) regulations, many waste generators are forced to store these solutions because current waste handling facilities are not permitted to accept them. It is estimated that world-wide usage of all LSC materials result in the generation of between 300,000 and 1 million gallons of waste per year, approximately one-third of which is produced in the United States. The current cost to the user of such liquids is about evenly divided between the purchase price and the disposal or storage cost. Other than long-term storage, incineration (at a very limited number of sites and at high cost) is the only disposal option.
In addition to the organic solvents, LSC liquids typically contains a relatively high quantity of emulsifier and a relatively small quantity of fluorescing agents. Typical emulsifiers include Triton-X 100.RTM. and other similar compounds sold by Rohm and Haas. Generally speaking, the LSC contains about 70% organic solvent and about 30% emulsifier. Flourescing agents include Eastman Kodak's bisMSB and PPO.
Microbial degradation of toluene, xylene and pseudocumene has been demonstrated by Pseudomonas putida and P. paucimobilis. However, such biodegradation has been demonstrated to occur only in a batch type process, and only with relatively low concentrations of the organic solvents. Thus far, a viable continuous process utilizing a microorganism capable of long-term exposure to such solvents has not been available, especially in the presence of high levels of emulsifiers.
The concentration of the total organic solvents in a stored quantity of the LSC will typically be on the order of from about 50% to about 90% of the solution, with the emulsifier comprising from about 10% to about 50% of the solution. Most commonly, the LSC solution contains organic solvent and emulsifier in a proportion of about 70:30. In the biodegradation of LSC, it would be advantageous to be able to use such stored liquids without the necessity of undergoing a substantial dilution to avoid microorganism toxicity, which would increase both the time necessary for biodegradation and the size of the bioreactor.
The advantages of a continuous bioreactor or chemostat, as opposed to a batch-type reactor, can be significant. A continuous bioprocessing system can maintain the optimum conditions for living cells. Since the microorganisms are the catalysts in the process, optimum conditions enable maximum rates of biodegradation. Factors contributing to the advantages of continuous processing include: maintenance of high viable cell densities; optimum substrate concentrations, and continuous removal of by-products which can inhibit further degradation. Additionally, a continuous process offers a significant manpower and cost savings in that it can be "turned on" and thereafter substantially ignored. By contrast, batch-type process require regular monitoring of microbial viability and consequent biodegradation, as well as periodic recharging of the bioreactor with nutrients, microorganisms and LSC waste product to be degraded.
Therefore, there is a need among users of LSC for an LSC waste-treatment unit which operates on a continuous basis, produces non-toxic effluent, and is capable of digesting relatively high concentrations of the organic LSC solvents, especially in the presence of high levels of emulsifier.